Publications Repository - Helmholtz-Zentrum Dresden-Rossendorf

Spin-transfer torques (STTs) can be exploited in order to manipulate magnetic moments of nanomagnets, allowing for new consumer-oriented devices to be designed, such as tuneable radio-frequency spin-torque nano oscillators (STNOs) for wireless communication. Currently, the structure involving an MgO-based magnetic tunnel junction (MTJ) with hybrid geometry combining an IP reference layer and an out-of-plane free layer is the system of choice [1,2]. This configuration, Fig. 1a, maximizes the output power, reduces the critical current [3], and can allow for stable precession regardless of magnetic or applied current history [1,4,5].

Here, we experimentally observe an unusual curvature of the critical lines on the current-field phase diagram enclosing the region of steady-state dynamics (Fig. 2a) which has never been reported in similar metallic- or MTJ-based devices. Theoretically, we incorporate the angular dependence of the TMR (dRAP/dV) [6-8] and bias dependent spin-transfer torques [9-11] into the in-plane and the perpendicular STT and solve LLGS equation [12]. We find that the angular dependence of TMR introduces an asymmetry in the in-plane STT and gives rise to stable precession. Moreover, including the bias dependence of TMR (Fig. 1b) correctly reproduces the curvature of the dynamical region in the experimental phase diagram (Fig. 2b), gradually suppressing the induced asymmetry, and with it the output power. Therefore, the TMR ratio and its bias dependence are both equally crucial factors governing the performance of MTJ-based STNOs.